The present invention generally relates to an electric heater for a rotating sample vessel container in a sampling device for gas chromatography and, in particular, relates to an electric heater which is stationary with respect to the rotating sample vessel container.
The sampling device is preferably of the type wherein a capillary connecting conduit formed as a needle is connected to the inlet of the separating column for the purpose of withdrawing samples from sample vessels which are closed by self-sealing membranes. A controllable valve is arranged in a carrier gas conduit leading to the inlet of the separating column. The valve may be closed for the purpose of sample feeding. The sampling device includes a table mounted on an axle and thermally controlled means for accommodating a plurality of sample vessels are provided such that each of the sample vessels is positionable under the stationary needle. Together the table and sample vessels are axially movable upwards in the direction towards the needle, whereby the sample vessel is pushed up on the needle such that the needle pierces the membrane.
One sampling device is known from German Auslegeschrift No. 1 297 904. In such sampling devices vapor from the head space above a sample enclosed in a sample vessel by means of a self-sealing membrane is supplied to the gas chromatograph. In the sample vessel closed by the membrane a state of equilibrium exists such that, in the head space above the sample, the different sample components are present with partial pressures, which are representative of the composition of the sample. A sample component, which is contained in high concentration in the sample, has a high partial pressure in the state of equilibrium in the head space. To obtain unambiguous results, the sample has to be maintained at a well-defined, relatively high temperature.
A sampling device for gas chromatographs is known from German Auslegeschrift No. 1 297 904. Therein, a turntable accommodating the sampling devices includes a thermostatized liquid bath. The sample vessels are inserted into the liquid bath from above. The liquid of the liquid bath evaporates. The turntable, which is rotatable relative to the liquid bath, is supported on a sealing ledge provided at the edge of the liquid bath. Furthermore, the liquid bath and the edge of the turntable accommodating the sample vessels are provided with a covering which is screwed on at the edge of the liquid bath. The covering is provided with arcuate openings through which the sample vessels can be inserted into the turntable. These openings are adapted to be closed by detachable lids. This arrangement is complicated in construction and manipulation.
German Offenlegungsschrift No. 22 44 260 discloses a device for preparing a series of sample preparations of physiological liquids with a transporting disc. Therein the device intended for increasing the temperature is provided in the area of the transporting disc. Sample vessels are guided in a holding groove extending along the circumference of the transporting disc. A heating liquid from a container is pumped through a heating device into the holding groove and exits via an overflow back into the container. Thus, the heating liquid flows around the sample vessels. Temperature sensors are arranged in the holding groove, by which the heating is controlled.
This is a sampling device for gas chromatographs with a turntable arranged to be lifted against a needle. Also, a liquid bath is provided as the thermostatizing means, only the liquid bath is heated and regulated in circulation. This circulation heater is expensive. Further, it is difficult to pass such a circulation through a rotatable turntable arranged to be lifted axially, as it is required in a sampling device of the above-mentioned type. In addition, when using a liquid bath the temperature range is limited by the boiling temperature of the liquid.
In German Offenlegungsschrift No. 28 18 251 an electrically heated metal block is provided as a thermostatizing means for the sample vessels. Therein, the metal block is rotatably mounted about a non-rotatable axle and has a circular array of axial through-bores with the axle as center. A base plate is non-rotatingly connected to the axle and closes the axial through-bores towards the bottom.
A closable aperture, allows for the insertion of a sample vessel into respective axial throughbores aligned with the aperture, is provided in the base plates such that the sample vessels can be inserted into the heated metal block by turning it. The axle with the base plate and the metal block is arranged to be moved in axial direction such that this axle is axially movable in the direction towards the needle.
The thermally controlled means, in the form of an electrical heater, includes an electric heating coil fixed to the metal block and is thus arranged to rotate therewith. Thus, the heating power in the form of electrical energy has to be transferred to the heating coil through sliders, which transference is disadvantageous in operational efficiency as well as being expensive to construct.